α-Hydroxycarboxylic acids and N-substituted aminocarboxylic acids are valuable building blocks for syntheses, and some are also utilized directly in various fields. For instance, 2-hydroxy-4-methylmercaptobutyric acid is used as an animal feed additive in a manner similar to methionine. On the industrial scale MHA is conventionally obtained from 3-methylmercaptopropionaldehyde, itself obtainable by an addition reaction between methylmercaptan and acrolein, by reaction with hydrogen cyanide followed by hydrolysis of the 4-methylmercapto-2-hydroxybutyronitrile which is formed.
Disadvantages of the last-mentioned process are the demanding safety requirements necessitated by the toxicity of hydrogen cyanide, and effluent pollution occasioned by the ammonium salt formed during the hydrolysis. Efforts to overcome the indicated disadvantages have brought to light processes in which carbon dioxide is reacted as a C1 building block with an aldehyde, ketone or imine to give the generic α-substituted carboxylic acid.
The electrochemical reaction of carbon dioxide with ketones and aldehydes with formation of α-hydroxycarboxylic acids is known from EP-A 0 189 120 and GDCH Monograph, Vol. 23 (2001), pp. 251-258. While the electrochemical carboxylation of aromatic ketones results in average-to-good yields, yields from the electrochemical carboxylation of aromatic aldehydes are moderate and those from the carboxylation of aliphatic aldehydes are low. In these cases the electrocarboxylation takes place in an undivided electrolytic cell in the presence of a sacrificial anode in an aprotic solvent which additionally contains a conducting salt.
According to WO 02/16671 an electrocarboxylation which works in accordance with the proposed principle is that of 3-methylmercaptopropionaldehyde (MMP) to obtain the methionine hydroxy analogue (MHA). When the electrolytic conditions indicated in WO 02/16671 were applied to the electrolysis in a continuous flow electrolytic cell having a plane-parallel electrode configuration, it emerged that the current efficiencies and material yields indicated in WO 02/16671 could not be attained. The current efficiencies achieved under the conditions of WO 02/16671 in this industrially more attractive cell design having a plane-parallel configuration of an Mg anode and an Mg cathode were around 13%, and the material yields were around 19%. According to a lecture by Reufer on the occasion of the 5th International Workshop on Diamond Electrodes (May 6, 2002-Jul 6, 2002, Itzehoe) this process can be improved by using as the cathode a planar boron-doped diamond electrode and as the anode an Mg sacrificial anode.
With the use of dimethylformamide as the solvent and tetrabutylammonium tetrafluoroborate as the conducting salt and with carboxylation carried out at a current density of 6 mA/cm2, in electrolysis with an Mg sacrificial anode and a diamond film cathode an MMP conversion of 66% and a current efficiency of 22% with reference to MHA formed were obtained.
In a manner analogous to the carboxylation of MMP according to WO 02/16671 A1, according to DE 100 40 401 A1, N-substituted imines can undergo cathodic carboxylation to N-substituted α-amino acids. The disadvantage here, as in the process which is acknowledged above, is the necessary use of a sacrificial anode.